Electroluminescent devices are used in a variety of low power applications. For example, many battery-operated devices, such as wrist watches, utilize electroluminescent illuminators to illuminate their displays. In such devices, energy for activating the electroluminescent lamp typically is provided by a battery at a voltage range of 1-9 V. However, for proper operation, electroluminescent lamps operate at much higher voltages, on the order of 50-300 V. Thus, typical battery output voltages are inadequate for directly driving electroluminescent lamps.
Another limitation of battery sources is that batteries supply DC voltages, while electroluminescent displays typically emit light responsive to AC voltages. To improve the lifetime of the lamp and for acceptable performance, the driving voltage for the electrolimrlinescent lamp preferably approximates a sine wave.
A variety of circuit structures have been suggested for driving electroluminescent lamps using batteries as power sources. For example, U.S. Pat. No. 5,349,269 to Kimball employs two inverters that each supply high frequency pulses to a lamp. To improve the efficiency of the lamp, each of the inverters drives an opposite terminal of the lamp so that the overall voltage change across the lamp is twice the voltage of pulses from each of the inverters. Unfortunately, the use of two inverters increases the cost and difficulty of fabrication of the driving circuit, in part because each of the inverters requires a separate inductor.
In another approach described in U.S. Pat. No. 4,527,096 to Kindlmann, a single inductor is used to drive opposite sides of an electroluminescent lamp through a switching network. The inductor drives a first side of the electroluminescent lamp while the second side of the lamp is coupled to ground. The inductor injects current to the first side of the lamp thereby charging the lamp to high voltage. Then, the lamp is discharged by coupling the first side to ground.
The switching network then couples the inductor to the second side and the inductor charges the second side. The second side is then grounded, once again discharging the lamp. The resulting waveform across the electroluminescent lamp is a sawtooth wave having positive and negative peaks. The sawtooth waveform deviates substantially from the preferred sinusoidal waveform and produces a rapid discharge of the electroluminescent lamp at a relatively high current. This rapid discharge can cause premature failure of the lamp and typically does not provide optimum current draw or efficiency.